Process for purifying phthalide

ABSTRACT

The disclosure is a process for purifying phthalide contaminated by carboxylic acids and/or their derivatives, which comprises adding polyethylene glycol and/or polypropylene glycol to the phthalide to be purified and then performing a fractional distillation.

This application is a 371 of PCT/EP97/03045 filed Jun. 12, 1997.

The present invention relates to a process for purifying a phthalidecontaminated by carboxylic acids, carboxylic anhydrides and/orcarboxylic esters.

Various syntheses have been described for the preparation of phthalidestarting from phthalic acid derivatives. In general, phthalide isprepared by selective hydrogenation of phthalic anhydride (DE-C-28 03319).

The problem is generally the work-up and purification of theas-synthesized reaction products, since phthalide synthesis almostinevitably produces a crude product contaminated by carboxylic acids andtheir derivatives such as phthalic acid, phthalic anhydride, monomethylphthalate, tetrahydrophthalide and 2-methylbenzoic acid. Removal ofthese by-products is very costly and sometimes not possible just bydistillation.

It is known that phthalic anhydride and monomethyl phthalate inparticular are not completely removable from phthalide by distillation.If the crude product contains phthalic acid, additional phthalicanhydride is formed during the distillation.

The problems with the removal of by-products mean that phthalide, theproduct of value, is obtained in only unsatisfactory yield and purity ina distillative work-up.

DE-C-32 45 544 discloses a process for distillative purification ofphthalide contaminated by carboxylic acids by admixing the phthalide tobe purified with n-butanol or isobutanol prior to the distillation andheating the mixture in an autoclave at between 150 and 280° C. forseveral hours before subjecting it to a fractional distillation.However, the disadvantages of this procedure are that it is verytime-consuming and that excess butanol has to be distilled off prior tothe distillation of the phthalide.

It is an object of the present invention to provide a simple andimproved process for purifying phthalide essentially contaminated bycarboxylic acids and their derivatives.

We have found that this object is advantageously achieved by a processfor purifying phthalide contaminated by carboxylic acids, carboxylicanhydrides and/or carboxylic esters, which comprises adding polyethyleneglycol and/or polypropylene glycol to the phthalide to be purified andthen performing a fractional distillation.

In general, the polyethylene glycol and/or polypropylene glycol is orare added to the phthalide to be purified in amounts from 1 to 50% byweight, advantageously from 2 to 40% by weight, preferably from 3 to 30%by weight, especially from 5 to 15% by weight, particularlyadvantageously from 8 to 12% by weight, based on phthalide.

It is advantageous to use polyethylene glycol and polypropylene glycolwhose average molar mass is within the range from 200 to 600, preferablywithin the range from 250 to 400.

The polyethylene glycols are advantageously obtained by polyaddition ofethylene oxide to ethylene glycol as initiator molecule in systemsusually containing small amounts of water. The polypropylene glycols areformed in a corresponding manner by polyaddition of propylene oxide to1,2-propanediol as initiator molecule.

It is an advantage of the process of the invention that, in general,even the addition of a relatively small amount of polyethylene glycol orpolypropylene glycol to the crude phthalide product is sufficient forthe purification process. It is a further advantage of the process thatit is possible to dispense with a time-consuming thermal pretreatment ofthe admixture of crude phthalide and polyethylene glycol orpolypropylene glycol. There is no need for prior heating, for exampleunder reflux or in an autoclave. Accordingly, in a preferred embodimentof the purification process, the addition of polyethylene glycol orpolypropylene glycol to the crude phthalide is followed by thefractional distillation directly, without further pretreatment. If thechemical structure and the molar mass of the polyethylene glycol orpolypropylene glycol are selected in such a way that this ancillarycomponent does not include anything which boils lower than phthalide,phthalide would be one of the low boilers in the distillation system. Onapplication of this preferred procedure, unconverted polyethylene glycolor polypropylene glycol and their reaction products remain behind in thebottom product of the fractional distillation.

The phthalide obtained by the process of this invention is notable forparticularly good purity and it is also obtained in good yield, theyield achieved being higher than the yield achieved via a distillationwithout addition of polyethylene glycol or polypropylene glycol. Thepolyethylene glycol or polypropylene glycol remaining behind in thebottom product also means that the bottom product remains liquid. Thisis advantageous, on the one hand, for the removal of the distillationresidue from the distillation flask and, on the other, for a preferredprocedure in which, after completion of each fractional distillation ofcrude phthalide to be purified, the distillation flask is repeatedlyrefilled with crude phthalide for another fractional distillationwithout the distillation residue from the preceding fractionaldistillation being removed.

Phthalide is an important intermediate, for example for the synthesis ofcrop protection agents.

The Examples which follow illustrate the invention.

INVENTIVE EXAMPLE 1

The fractional distillation was carried out using 1600 g of crudephthalide solution having approximately the following composition: 46.0%by weight of butyrolactone, 34.9% of phthalide, 15.0% by weight ofwater, 2.5% by weight of 2-methylbenzoic acid, 1.0% by weight ofphthalic anhydride, 0.3% by weight of tetrahydrophthalide and 0.3% byweight of further by-products. This crude phthalide solution was admixedwith 176 g (32% by weight, based on phthalide) of a polyethylene glycolhaving an average molar mass of 300. Then, without any furtherpretreatment, a fractional distillation was carried out using alaboratory column having 40 theoretical plates and a top-of-columnpressure of 20 mbar. The distillation yield was 73% of phthalide in apurity of more than 98.5%.

The distillation residue was subsequently not removed for disposal, butit remained in the distillation flask. By repeatedly filling the flaskwith further crude phthalide solution and repeating the distillation,the total phthalide yield was increased to 87%. The phthalic anhydridecontent of the pure phthalide product was below 0.1%, and the2-methylbenzoic acid content was below 0.2%.

INVENTIVE EXAMPLE 2

The fractional distillation was carried out similarly to InventiveExample 1, except that only 9% by weight, based on pthalide, ofpolyethylene glycol of the average molar mass of 300 was added to thecrude phthalide solution prior to the distillation. The phthalideobtained in a yield of 87% had a purity of more than 98.5%, its phthalicanhydride content being below 0.1% and the 2-methylbenzoic acid contentbelow 0.3%.

COMPARATIVE EXAMPLE

The fractional distillation was carried out using 1600 g of a crudephthalide solution of the same composition as in Inventive Example 1.The crude phthalide solution was distilled through a laboratory columnhaving 40 theoretical plates using a top-of-column pressure of 20 mbar.The distillation yield was 58% of phthalide, obtained in a purity of97.6% and with a phthalic anhydride content of 1.0%. A phthalide productof this purity is unsuitable for most applications.

We claim:
 1. A process for purifying phthalide contaminated bycarboxylic acids, carboxylic anhydrides and/or carboxylic esters, whichcomprises adding polyethylene glycol and/or polypropylene glycol to thephthalide to be purified and then performing a fractional distillation.2. A process as claimed in claim 1, wherein the polyethylene glycoland/or polypropylene glycol added to the phthalide to be purified has anaverage molar mass within the range from 200 to
 600. 3. A process asclaimed in claim 1, wherein the polyethylene glycol and/or polypropyleneglycol added to the phthalide to be purified has an average molar masswithin the range from 250 to
 400. 4. A process as claimed in claim 1,wherein the amount of polyethylene glycol and/or polypropylene glycoladded to the phthalide to be purified is within the range from 1 to 50%by weight, based on phthalide.
 5. A process as claimed in claim 1,wherein 2-40% by weight of polyethylene glycol is added.
 6. A process asclaimed in claim 1, wherein the fractional distillation is performeddirectly following addition of polyethylene glycol and/or polypropyleneglycol to the phthalide to be purified and is not preceded by furtherpretreatment.
 7. A process as claimed in claim 1, wherein thepolyethylene glycol and/or polypropylene glycol added does not includeanything which boils lower than phthalide.
 8. A process as claimed inclaim 1, wherein unconverted polyethylene glycol and/or polypropyleneglycol remain behind in the bottom product of the fractionaldistillation and phthalide is distilled off as low boiler.
 9. A processas claimed in claim 8, wherein, after each completed distillation, thebottom product remains behind in the distillation flask and thedistillation flask is refilled with crude phthalide.
 10. A process asclaimed in claim 1, providing phthalide in a purity of more than 98.5%.